Bi-directional reflectance distribution function (“BRDF”) gives the reflectance of a target as a function of illumination geometry and viewing geometry. The BRDF depends on wavelength and is determined by the structural and optical properties of the surface. Optical and structural properties may include shadow-casting, multiple scattering, mutual shadowing, transmission, reflection, absorption and emission by surface elements, facet orientation distribution and facet density.
BRDF can be determined, for instance, for clouds, land cover, and radiometric boundaries, although the present disclosure is directed at determining the BRDF of samples or subjects that can be fit within an enclosure. Examples of these samples or subjects may include, for example, films or materials extracted from objects for study. The samples may generally have a small footprint. A number of devices exist for determining the BRDF of samples of this size.
One type of known device for determining the BRDF is a device that is known in the industry as a 2D BRDF. As the name implies, the BRDF is measured in two dimensions. However, to completely appreciate the BRDF results, a 3D image must be provided. A composite 3D image can be created, but it requires integrating many 2D images and recording the many 2D images is time intensive.
There also are 3D BRDF devices available in the marketplace. One such 3D BRDF device includes directing a collimated light beam against a sample within a white hemispherical screen and capturing an image of the scattered light pattern produced on the hemispherical screen by the sample. A problem with this system is that the white screen tends to re-scatter light onto other parts of the screen, which diminishes the reliability and accuracy of the 3D BRDF result. This secondary scattering of light from one part of the screen to other parts of the screen is known as cross talk. This approach also suffers from very low optical efficiency. Most of the light that is scattered off of the white screen will not be received by the sensor. This will either result in excessively long signal integration times of samples which exhibit very low BRDF, or will prevent measurement of low BRDF samples.
Other known 3D BRDF devices do not include an imaging screen. These other known 3D BRDF devices are significantly more expensive while producing results that are not materially more reliable than the aforementioned 3D BRDF with a white hemispherical screen.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.